The invention relates to an optical device in which electromagnetic radiation is raised in frequency, which device comprises a diode laser for supplying radiation to be raised in frequency and a non-linear optical medium having an acceptance bandwidth, which medium ensures the frequency raise.
The invention also relates to an apparatus for optically scanning an information plane, comprising such a device.
The acceptance bandwidth is understood to mean the width of the wavelength band, around a nominal wavelength, of the radiation which can efficiently be raised in frequency by the non-linear optical medium.
Scanning may be understood to mean both scanning during writing and scanning during reading an information plane of, for example, an optical record carrier.
A device of the type described in the opening paragraph is known from the publication "Blue-light laser ups CD density" in "Electronics" August 1988, page 48. The device described in this publication is used in an apparatus for reading an optical record carder in which an audio program is stored. By doubling the frequency, or in other words, halving the wavelength, of the radiation of a conventional diode laser, the diameter of the read spot formed by this radiation can be halved. Consequently, information details can be read whose dimensions are half those of the information details which can be read without using frequency doubling. The frequency doubling has the great advantage that the information density in optical record carriers can be considerably increased, for example by a factor of four.
To improve the efficiency of the frequency raise, use has to be made of a relatively long non-linear optical medium. Most media have, however, a relatively small acceptance bandwidth so that relatively stringent requirements must be imposed on the diode laser in such devices. The most important requirements are:
1) The wavelength band of the radiation emitted by the diode laser is to be within the acceptance bandwidth of the non-linear optical material. PA1 2) The diode laser must have a very stable emission wavelength so that this wavelength is always within the acceptance bandwidth of the non-linear optical medium. This implies that the output spectrum of the diode laser must not change. PA1 the repetition frequency of the diode laser pulses; PA1 the optical path length of the radiation pulses returning to the diode laser; PA1 the energy of the radiation pulses returning to the diode laser; PA1 the acceptance band of the non-linear optical medium.
This requirement restricts the output of usable diode lasers to a considerable extent.
Particularly the latter requirement may be difficult to meet in practice because both the diode laser and the non-linear optical medium must be stabilized in temperature very accurately, for example, up to 0.5.degree. C., because both the diode laser and said medium have a strong temperature-dependent behaviour.
If temperature variations lead to a change of the output spectrum of the diode laser, giving its radiation a different wavelength, substantially no radiation with a doubled or raised frequency is emitted from the non-linear optical medium and the device has become ineffective.